1. Field of the Invention
The present invention generally relates to the analysis of paper surface qualities. More particularly, the invention relates to a method and apparatus for three dimensionally measuring and evaluating data descriptive of a paper surface topography.
2. Description of the Prior Art
As applied to paper, "print quality" is a subjective, human eye assessment of a particular paper surface respective to the printed product of that surface. One of the more influential objective factors bearing upon the print quality of a paper surface is the "roughness" of that surface. Additionally, paper is also subjectively appraised for simply unprinted visual surface smoothness. Consequently, numerous methods have been devised to measure the roughness of an unprinted paper surface as a predictor of the printed product.
Several widely used, indirect, methods of paper surface roughness measurement, characterized as Sheffield smoothness, Bekk smoothness and Parker Print Surf, include air leak techniques which measure the volume of air that leaks, over a fixed time interval, between the paper surface and a test equipment seal, or, conversely, the time required for the leakage of a fixed air volume. Although these indirect surface roughness measurement methods offer a numerically quantified, relative value of surface roughness, they have not proven to be consistent, reliable indicators of print quality.
Traditional surface roughness measurement relies upon the single, line trace of a stylus over the surface as is represented by U.S. Pat. No. 4,888,983 to L. G. Dunfield et al. Surface height values are measured at uniformly separated increments within a single, vertical evaluation plane. Such surface height values are digitized and processed algorithmically by Dunfield et al. to determine a corresponding print quality index.
Machine made paper, however, has a directional orientation relative to the machine production line. A greater percentage of papermaking fiber aligns with the machine direction than with the cross-machine direction. Extremely exaggerated, this circumstance of machine direction fiber alignment may be perceived as a corrugated topography. If measured by a single, vertical displacement stylus in a direction transversely of the corrugation ribs, the analysis would report a "rough" surface with large but highly regular vertical distance variations between the measured peaks and valleys. When measured in a direction parallel with the corrugation ribs, the analysis would report a "smooth" surface with small variations between vertical distance variations. Only by means of three dimensional measurement may the parallel ribbed nature of the surface be recognized.
From the foregoing illustration, it is demonstrated that a three dimensional topography is defined by length, width, and height coordinates, the length and width coordinates being in the same plane and the height coordinate measured perpendicularly from the length/width plane.
It is, therefore, an object of the present invention to provide a method and apparatus for quantifying the roughness quality of a paper surface based upon the three dimensional topography of the surface.
Another object of the present invention is to provide a method and apparatus for paper surface roughness measurement which accommodates the surface directionality.
Another object of the invention is to provide a digital characterization of a surface topography.